1. Field of the Invention
The present invention relates to a drive apparatus for driving an oscillator with a resonance frequency and a powder feeder having the drive apparatus therein. In particular, the present invention relates to a drive apparatus for driving an oscillator with a resonance frequency, in which Phase Lock Loop (PLL) control is conducted to follow the resonance frequency given to the oscillator with an actual resonance frequency when the resonance frequency of the oscillator having the resonance frequency actually changes, and relates to a powder feeder in which the drive apparatus is installed.
2. Description of Related Art
In a case of controlling an oscillator with a resonance frequency in the resonance region (resonance point), it is conventionally popularized a control method for controlling a driving voltage given to the oscillator. For example, it is shown in FIG. 10 a driving voltage control circuit for driving an ultrasonic motor. In this control circuit, a peak value of the driving voltage is controlled in a DC-DC converter.
Operation of the above control circuit will be shown in FIG. 12 by indicating a relationship with the driving voltage supplied to the ultrasonic motor.
On the other hand, in case of driving the oscillator with the resonance frequency in the resonance region (resonance point ) to continuously drive it as in the method by the driving voltage control circuit, it is difficult to precisely control the output such as vibration amplitude of the oscillator in the resonance point by the driving force (for example, the driving voltage ). Further, there is a problem that feedback cannot be done in a narrow control region.
Thus, it is proposed a control method to intermittently apply the driving force to the oscillator and control it. For instance, there is a control method that a driving voltage is applied intermittently to control the operation time per one cycle (duty ratio) namely the time average output. Such control is performed, for example, using a circuit shown in the block diagram of FIG. 11.
As a motor capable of such controlling, for example, an ultrasonic motor using an ultrasonic resonator is known. In the ultrasonic motor, the mechanical deformation of a piezoelectric element caused by electric energy is used to generate mechanical vibration of a vibrator and the output of the ultrasonic motor is changed by changing the duty ratio of the driving voltage.
For instance, an ultrasonic resonator which generates both axial vibration (longitudinal vibration) and bending vibration generates elliptic oscillation at a top end thereof with the resonance frequency. A pipe is attached to the top end of the resonator, and powder is fed in the pipe, then the powder is moved in the certain direction, this mechanism can therefore be used as a powder feeder. In this case, also an AC driving voltage with the resonance frequency is applied to the resonator intermittently to control the feed amount of the powder. The driving voltage controlled by the duty ratio is shown in FIG. 13.
In some cases, driving force having resonance frequency is applied intermittently in order to obtain pulse vibration. In the case of a fish detector for investigating topography of sea floor or fish by transmitting ultrasonic waves into water and by detecting reflected echos, a driving voltage of a resonance frequency is applied intermittently into water to transmit ultrasonic waves into water. On the other hand, after the transmission of ultrasonic waves the vibration is stopped, and an echo is received from water and thus the fish detector serves as a sensor for catching the information in water.
Similar examples include an ultrasonic wave sensor for detecting the existence of some objects in air by emitting ultrasonic waves into water and detecting reflected ultrasonic waves from an object and an ultrasonic range finder for measuring the distance by measuring reflection time of the ultrasonic waves.
On the contrary, there will be a case that the resonance frequency of the vibrator, for example, in the powder feeder, changes on the basis of change in weight of the powder while feeding thereof. In cases that the resonance frequency of the vibrator actually changes, it is widely used a Phase Lock Loop (PLL) control circuit as a control circuit to follow the resonance frequency of the vibrator with the actual resonance frequency. A general PLL control circuit is shown in FIG. 4. Operation of the PLL control circuit is shown in FIG. 5.
The PLL control circuit has a feedback loop utilized for extracting (demodulating) a base band signal from a frequency-modulated carrier wave. The PLL control circuit is constructed from a phase comparator 101, a loop filter 102 and a voltage control oscillator 103, as shown in FIG. 4. In the PLL control circuit, a phase of the input signal and a phase of output signal from the voltage control oscillator 103 are mutually compared in the phase comparator 101, and the output from the phase comparator 101 is input to the loop filter 102. Further, based on the output from the loop filter 102, the frequency of the voltage control oscillator 103 is controlled.
That is, if the frequency of the input signal and the frequency of the voltage control oscillator 103 are different, a beat signal corresponding to difference between the frequencies of the input signal and the voltage control oscillator 103 is produced as the output signal of the phase comparator 101. In FIG. 5, if the output signal lies in a range of synchronism in the PLL control circuit, the frequency of the voltage control oscillator 103 approaches to the frequency of the input signal in the positive half-period, and goes away from the frequency of the input signal in the negative half-period. Based on this, the DC component changes more slowly in the positive half-period than in the negative half-period, and the level of DC component becomes totally positive. The voltage control oscillator 103 is controlled so that the difference between the frequencies becomes smaller by the DC voltage. Both the frequencies of the input signal and the voltage control oscillator 103 completely synchronize when the response of the PLL control circuit can follow with the wave of the beat signal.
However, there exist the following problems in the above conventional drive apparatus for the oscillator. In the voltage control method, if the peak value of the driving voltage becomes low, it becomes difficult to detect the current in the phase comparator 101 of the PLL control circuit. Further, the PLL control circuit is opened when correct voltage is not applied to the vibrator, such as the ultrasonic motor. The resonance frequency of the vibrator therefore cannot follow with the actual resonance frequency when the resonance frequency of the vibrator actually changes.
Also, in the duty ratio control method, the PLL control circuit is opened in an inactive period of the duty ratio, and the resonance frequency of the vibrator cannot follow with the actual resonance frequency when the resonance frequency actually changes. In particular, this problem becomes remarkable when the duty ratio is small. Here, signal waves in the circuit shown in FIG. 11 are shown in FIG. 14. FIG. 14(a) shows an output signal from the duty ratio control circuit, FIG. 14(b) shows an output signal from the drive circuit, and FIG. 14(c) shows an output signal after waveform shaping. After waveform shaping, the output signal for the pulse corresponding to a period during which vibration is not given to the vibrator by the duty ratio control circuit vanishes. Therefore, feedback is not conducted in the PLL control circuit and the PLL control circuit is opened. As a result, the PLL control circuit does not operate when the resonance frequency of the vibrator actually changes.
Further, in the resonators of the fish detector or the ultrasonic range finder, there remains a problem that the time difference measurement between the emitted and reflected ultrasonic waves is erroneously conducted when the frequency of ultrasonic waves is fluctuated.